Propulsion Trades for a 2035–2040 Solar Gravitational Lens Mission

Direct multipixel imaging and high-resolution spectroscopy of exoplanets with the Solar Gravitational Lens (SGL) requires operations on the focal line at z ≃650–900 AU, where solar power is negligible and transportation dominates time-to-first-science. Reaching 650 AU in 20 yr implies a mean radial speed ¯ vr = 32.5 AU yr−1 = 154 km s−1 (ballistic lower bound), motivating propulsion beyond chemical and gravity-assist-only options. We compare close-perihelion solar sailing, fission-powered nuclear electric propulsion (NEP), and Oberth-enabled hybrid injection using transparent time-to-distance models (excluding architecture-dependent inner-solar-system injection overhead). Solar sailing provides the highest near-term in-principle hyperbolic excess: at r_p = 0.05 AU, v_∞ = 105 km s−1 requires σ_tot ≃4.9 g m−2, while v_∞ = 155 km s−1 requires σ_tot ≃2.3 g m−2, placing sub-20 yr sail-only access in a combined low-areal-density and deep-perihelion survivability regime at mission scale. For NEP, constant-power stage closure shows that a m_0 = 20 t spacecraft with mpay = 800 kg, η= 0.7, and high-Isp propulsion reaches z = 650 AU in∼27–33 yr for αtot = 10–20 kg kW^−1 e (typical optima: Pe ≃0.18–0.30 MWe, mp ≃15–17 t, thrust of only a few newtons). NEP-only sub-20 yr transfers require extremely aggressive system assumptions (αtot ≲ 3 kg kW^−1 e and very high-voltage, long-life EP), whereas hybrid architectures become plausible for αtot ∼10–15 kg kW^−1 e if an injection stage supplies v0 ≳ 50–70 km s−1 prior to NEP cruise. We map these requirements to technology readiness and programmatics and identify the system-level demonstrations needed by the early 2030s for a credible 2035–2040 start.